The present invention relates to moldable compositions based on thermoplastic polymers and vegetable fibrous materials. It also relates to the use of these compositions, which possess improved suitability for shaping by the conventional molding processes, for the thermoforming of articles from sheets or panels.
Moldable compositions based on thermoplastic polymers and vegetable fibrous materials, such as wood flour, are already known.
It is also known that the articles molded from these compositions exhibit advantageous properties, for example, good impact strength and good rigidity in the case of compositions based on propylene homopolymers and on wood (E. L. SOULE and H. E. HENDRICKSON, FOREST PRODUCTS JOURNAL, vol. 16, pages 17-22, 1966; German patent application No. 1,769,178 in the name of B.A.S.F.) or good impact strength and good resistance to moisture in the case of compositions based on polyethylene and on wood (German Pat. No. 1,241,981 in the name of RUHRCHEMIE).
However, these compositions themselves exhibit the serious disadvantage that they are difficult to process and to shape by the conventional techniques.
Thus, it has been found, on processing by injection techniques, that the injectability of these compositions was mediocre and that the high temperatures required frequently resulted in their degradation.
Furthermore, an important outlet for these compositions is the manufacture of shaped articles from extruded or calendered sheets or panels. However, when these compositions are calendered, it is found that the sheets tend to stick to the hot rolls. In addition, serious difficulties are encountered in the course of shaping these sheets, in particular when they are folded, stapled and the like. Finally, in the majority of cases, where vacuum thermoforming is carried out, the suction must be assisted by a male matching mold and the final appearance of the thermoformed articles nevertheless leaves something to be desired.
French Application No. 76,39191, available to the public July 22nd, 1977, discloses compositions containing 99 to 70 parts by weight of a crystalline polyolefin modified by grafting and 1 to 30 parts by weight of an elastomer. The incorporation of vegetable fibrous materials, such as sawdust, in these compositions is not envisioned.